That, Kovacs said, could shed new light on why the Soviets arrested Wallenberg in Budapest in 1945 and why supposedly neutral Sweden remained so passive following his disappearance.

“For me it is very clear that it was also Wallenberg’s mission to act as some kind of coordinator between the resistance forces and the Allies,” Kovacs told The Associated Press.

Other researchers investigating Wallenberg’s fate called the information significant, but said it wasn’t enough to conclude that Wallenberg gave military support to Hungarian resistance fighters.

It’s well known that Wallenberg’s work as Sweden’s envoy in Budapest was a cover for a humanitarian mission as secret emissary of the U.S. War Refugee Board, created in an attempt to stem the annihilation of Europe’s Jews. He saved at least 20,000 Jews in Budapest by giving them Swedish travel documents or moving them to safe houses.

In his book, whose Swedish title could be translated as “Dark skies over Budapest,” Kovacs says documents he found in Hungary’s military history archives show how a member of the resistance movement communicated the position of Nazi Germany’s ships in the Danube river to the allies via radio equipment in the Swedish embassy. British planes based on Malta then bombed the ships.

While there is no documentation that links those activities directly to Wallenberg, Kovacs says his research shows Wallenberg had frequent contacts with leaders of the non-communist resistance movement including Kalman Zsabka and Zoltan Miko. Swedish assistance to the Hungarian resistance movement in military operations with the allies would have run counter to Sweden’s neutrality.

Previous research has also shown Wallenberg was in contact with high-ranking resistance leader Geza Soos.

Part of Kovcac’s work is based on research by Hungarian historian Jozsef Gazsi during the Cold War. Gazsi interviewed several former members of the resistance movement who said they had met Wallenberg. One of them, Ferenc Kalmanffy, even said that Wallenberg had given them “hand-grenades, pistols and some machine guns,” according to Hungarian documents that Kovacs cites in his book.

Susanne Berger, a long-time Wallenberg researcher, called that information significant.

“This is clearly a military political activity and that really stirs up a whole new hornet’s nest,” she told AP.

“The sources and contents Gellert cites obviously have to be critically evaluated, but I see nothing in this material that would indicate that the alleged actions could not be true,” she added.

Swedish author Ingrid Carlberg, who published a biography of Wallenberg last year, said Kovac’s book “paints an entirely new and very interesting picture” of the resistance movement’s use of radio equipment in the Swedish embassy.

However, she said those activities were probably set in motion by the first secretary at the Swedish legation, Per Anger, who she said worked closely with the leader of the resistance group that used the radio equipment.

Wallenberg vanished after being arrested by the Red Army. The Soviets initially denied he was in their custody, but then said in 1957 that he died of a heart attack in prison on July 17, 1947.

If Wallenberg’s activities in Hungary extended beyond humanitarian work, that would make it easier to understand why the Soviets kept him in custody, Kovacs said.

“From their point of view it’s entirely rational,” he said. “They probably believed he had important information and saw him as a threat.”

The information also provides a broader context to Sweden’s passive reaction to Wallenberg’s disappearance, Kovacs said.

Sweden has been widely criticized for prioritizing its relations with the Soviet Union over finding out what happened to Wallenberg.

“He breached all existing diplomatic conventions. If he had made it back I think he would have been scolded by the Foreign Ministry and he would never have gotten another job there,” Kovacs said about Wallenberg. “I think this is the biggest reason why the foreign ministry was so feeble in the first years. They felt Wallenberg put the embassy at risk.”